The San Juan basin of northwestern New Mexico and southwestern Colorado is a Laramide structural basin with a maximum thickness of 15,000 ft (4,572 m) of Paleozoic to Eocene sedimentary rocks. The basin is elongate north-south, approximately 125 × 100 mi (201 × 161 km); it is structurally asymmetrical, with the deepest part in the north near the New Mexico-Colorado line. Monoclinal basin rims are especially prominent.

Petroleum occurs in Pennsylvanian carbonate rocks,

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Jurassic eolian sandstones, but predominantly in Cretaceous sandstones and fractured shales. The San Juan basin is gas-prone, although significant oil fields are common. The Cretaceous Blanco basin gas field is one of the largest in the United States. The key to exploration in this mature area lies in understanding the complex stratigraphy that controls most of the traps.

The San Juan basin area was a part of larger depositional basins prior to the Laramide orogeny. Major sedimentation in the area began during the Pennsylvanian with deposition of shelf-carbonates and flanking arkosic clastics from Ancestral Rocky Mountain uplifts. Regression during the Permian resulted largely in nonmarine clastic deposition. Triassic fluvial red beds were deposited across the area after an erosional interval. After another erosional interval, Jurassic nonmarine clastics and restricted limestones and evaporites were deposited. During the Cretaceous, the sea returned and repeatedly transgressed and regressed across the basin, producing well-developed depositional cycles. Laramide uplift around the San Juan basin during latest Cretaceous and earliest Cenozoic time produc d the structural basin which became partly filled with Paleocene and Eocene nonmarine clastics.

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